Method of reducing density by means of gas generating agent

ABSTRACT

This invention relates to a method of reducing density in an ammonium nitrate product. The method comprises providing a gas generating agent in the form of a water-insoluble solid compound capable of generating gas by chemical reaction and providing an ammonium nitrate product. The water-insoluble solid gas generating agent is introduction into the ammonium nitrate product under conditions causing gas generation by the gas generating agent in the ammonium nitrate product. The ammonium nitrate product is particulated to form a particulated prilled ammonium nitrate product with reduced density. The invention also relates to a particulated ammonium nitrate product produced by this method and to the use of a gas generating agent in the form of a water-insoluble solid compound reduce density in a particulate ammonium nitrate product.

TECHNICAL FIELD

This invention relates to a method of reducing density in a particulatedammonium nitrate product such as prilled or granulated ammonium nitrate,and to products formed by the method.

BACKGROUND ART

Particulated ammonium nitrate in the form of prilled ammonium nitrate,and particularly porous prilled ammonium nitrate (PPAN) is widely used,for example in the manufacture of explosive compositions. PPAN isusually formed by prilling an aqueous ammonium nitrate solution andthereafter drying the prills. During the drying process, the water inthe prills evaporates, creating the porosity in the prills to reducedensity and to allow absorption of fuels during the preparation ofexplosives. The process of drying the prills is expensive and the dryingapparatus used is also capital intensive.

In U.S. Pat. No. 5,540,793 encapsulated microspheres were introduced inthe prilled product to control the density and to render the fuelledprills more sensitive. The patent also teaches that the porous prilledproduct so formed may include limestone in fairly large amount from30.0% to 0.01% mass per mass final product. Some of the inventors ofU.S. Pat. No. 5,540,793 and the present invention are the same and it isknown to the present inventors that the limestone mentioned in U.S. Pat.No. 5,540,793 had a fairly large particle diameter of between 200 μm and500 μm and that it was used to desensitize the product.

When limestone is added in this manner, no gas generation takes placeduring the formation of the prills, mainly due to the large particlesize of the limestone but also due to factors such as acidity of theammonium nitrate and the timing of the addition of the limestone.

U.S. Pat. No. 5,540,793 also teaches that the porosity of the porousprilled product which includes the encapsulated microspheres can furtherbe enhanced by the inclusion of gas in the product during the prillingprocess. It is taught that this gas can be developed in situ in theproducts via a suitable chemical reaction. The gas may comprise carbondioxide which is formed through decomposition of a suitable carbonate inacid medium. The patent further teaches that the carbonate may compriseany suitable water-soluble inorganic salt of carbonic acid, for examplepotassium and/or sodium carbonate or alternatively, it may comprise aless soluble salt.

However, it was found that it is very difficult to control the rate ofthe gas release and bubble size when gas is developed in situ in thismanner. When the gas generation is too much the progressively formedcrust, resulting from the solidification of the ammonium nitrate on thesurface of the particle, will be too weak and the generated gas willdestroy the crust, resulting in excessive breakage of the particles.When gas generation is too low or too slow, the required low density andincreased porosity will not be developed.

It is believed the general trend is that the smaller the pores (down toabout 30 μm) in PPAN the higher will be the detonation velocity,detonation pressure, and thus the brisance of an explosive compositioncontaining such PPAN.

DISCLOSURE OF THE INVENTION

Surprisingly it has now been found that when a water-insoluble compound(capable of generating gas in a chemical reaction) is used and theconditions are suitable, for example by ensuring that the particle sizeof the compound is below a certain size, controlled gas release can beobtained in the formation of particulated ammonium nitrate to obtain aproduct of reduced density and a suitable pore size.

In this specification the term “water-insoluble compound” is used tomean a compound which dissolves in water at less than 1 g/100 g of waterat room temperature.

According to the present invention a method of reducing density in anammonium nitrate product comprises:

-   -   providing a gas generating agent in the form of a        water-insoluble solid compound capable of generating gas by        chemical reaction;    -   providing an ammonium nitrate product;    -   introducing the water-insoluble solid gas generating agent into        the ammonium nitrate under conditions causing gas generation by        the gas generating agent in the ammonium nitrate product; and    -   particulating the ammonium nitrate product to form a        particulated ammonium nitrate product with reduced density.

The particle density and the bulk density of the particulated ammoniumnitrate is reduced due to the formation of closed voids (also referredto as closed pores) within the particulated product. Some pores or voidsmay open to the outside of the particulated product; such open poreswill also reduce the density and will have the added advantage ofincreasing the porosity of the particulated product porosity ofparticulated ammonium nitrate is important for the absorption of fueloil in the manufacture of ANFO.)

It was found that when a water-insoluble solid gas generating agent isused, it is much easier to control the rate and amount of gas generatedduring the formation of the particulated product and to ensure that gasbubbles are formed with a suitable size to form pores of a suitablesize. Pores of a suitable size are preferably pores with a diametersmaller than 150 μm, preferably smaller than 70 μm, preferably about 30μm.

It will be appreciated that factors such as the type of gas generatingagent added; amount of gas generating agent added; particle size of thegas generating agent added; pH of the reaction mixture of the gasgenerating agent and ammonium nitrate product; pH of the ammoniumnitrate product; type and concentration of an acid provided to reactwith the gas generating agent (where the gas generating agent is of thetype which reacts with an acid to generate gas); type and concentrationof base provided in ammonium nitrate; the temperature at which the gasgeneration reaction takes place; timing of addition of the gasgenerating agent; the concentration of the ammonium nitrate, all play arole in the rate of gas generation, bubble size of the generated gas andtiming when the gas is generated. In use these factors can be controlledin order that the gas is generated during particulation of the product,that the gas is generated at a suitable rate and has a suitable bubblesize to form pores of a suitable size. Accordingly factors thatinfluence the rate of gas generation, bubble size of generated gas andtiming when gas is generated can be controlled to obtain the desiredeffect.

The gas generating agent may comprise a water-insoluble carbonatecompound, preferably a carbonate salt. Preferably it comprises a metalcarbonate, preferably an alkaline earth metal carbonate. Preferably itcomprises at least one compound selected from the group consisting ofcalcium carbonate and magnesium carbonate. Most preferably it comprisescalcium carbonate.

The gas generating agent may comprise a compound capable of generatinggas, preferably carbon dioxide (CO₂), by chemical reaction with asuitable acid. Calcuim carbonate is such a gas generating agent.Preferably the acid is an acid present in the ammonium nitrate product.

The average particle diameter of the water-insoluble gas generatingagent is preferably below 100 μm. More preferably it is below 40 μm,preferably below 2 μm, preferably below 1 μm and preferably below 0.5μm. In a preferred embodiment the average particle diameter is about0.33 μm. In a preferred embodiment of the invention the gas generatingagent comprises precipitated calcium carbonate preferably with anaverage particle diameter of about 0.33 μm. It has been found that theconditions for in situ formation of the gas can particularly becontrolled by providing the gas generating agent with a particlediameter as set out above. The particle diameter ranges described aboveof below 40 μm are especially (but not limited thereto) suitable forcalcium carbonate.

The gas generating agent is preferably provided in the form of anaqueous suspension. Preferably the suspension comprises between 1% and70% mass gas generating agent/total mass suspension, preferably about30% mass gas generating agent/total mass suspension, especially in thecase of calcium carbonate.

In the preferred embodiment of the invention where the gas generatingagent comprises precipitated calcium carbonate with an average particlediameter of about 0.33 μm, the calcium carbonate is added to theammonium nitrate product at a concentration of 0.001% to 2% mass calciumcarbonate/mass ammonium nitrate product. Preferably the calciumcarbonate is added at a concentration of 0.01% to 0.06% mass calciumcarbonate/mass ammonium nitrate, and preferably of a concentration ofabout 0.03% mass calcium carbonate/mass ammonium nitrate.

The ammonium nitrate product to be particulated may comprise a melt ofammonium nitrate.

Alternatively, the ammonium nitrate product to be particulated maycomprise an aqueous solution of ammonium nitrate. The aqueous solutionof ammonium nitrate may comprise above 90%, preferably above 96% massammonium nitrate/total mass aqueous ammonium nitrate solution. Toprevent ammonium nitrate solutions at such high concentration fromsolidifying and to facilitate particulation, the temperature of thesolution has to be raised. For example, in the case of 97% mass ammoniumnitrate/mass aqueous ammonium nitrate solution a temperature of about158° C. is required to prevent solidification and to facilitateparticulation. In the case of 99% mass ammonium nitrate/mass aqueousammonium nitrate solution a temperature of about 170° C. is required toprevent solidification and to facilitate particulation.

Preferably the ammonium nitrate product is acidic, preferably with a pHbetween 2 and 5, preferably about 4. This is especially the case wherethe gas generating agent is of the type which generates gas by reactionwith a suitable acid. However, it is foreseen that in other embodimentsthe solution may be alkaline. For example it is believed that a gasgenerating agent such as CaCO₃ will release NH₃ from ammonium nitrate,especially from ammonium nitrate with a high alkalinity. Alternativelythe ammonia could be generated by adding gas generating agents such ascalcium oxide to the ammonium nitrate.

The gas generating agent may be introduced into the ammonium nitrateproduct prior to particulation of the ammonium nitrate product.Alternatively it may be introduced during particulation of the ammoniumnitrate.

The ammonium nitrate product may be particulated by means of prilling.Alternatively it may be granulated, for example by using a pangranulator.

It is believed that the gas generating agent will be useful in moreconcentrated solutions where less water is available to evaporate andconsequently to create porosity.

Hollow microspheres (also known as microballoons) may also be used incombination with the gas generating agent. The hollow microspheres maybe used to increase the detonation efficiency, sensitivity and velocityof detonation of fuelled prills. Any type of hollow microsphere may beused but preferably hollow microspheres are used which do not contain acomposition which may decompose at high temperatures to form an acid.One such composition is polyvinylidene chloride (PVDC) which decomposesat high temperatures to form hydrochloric acid. An increase in theamount of acid in the product to be particulated will increase the rateof gas generation in cases where the gas generating agent is of the typewhich generates gas upon reaction with an acid. Furthermore, an increasein the amount of acid present in ammonium nitrate causes thedecomposition temperature of the ammonium nitrate to be lowered. At alow pH the decomposition temperature of ammonium nitrate is lowered tosuch an extent that it becomes unstable which may result in deflagrationor detonation.

Low density chlorine-free hollow microspheres, that are available incertain grades of Expancel (supplied by Akzo Nobel of Sweden), arepreferred hollow microspheres to be used in this invention.

The invention also relates to the use of a gas generating agent in theform of a water-insoluble solid compound capable of generating gas bychemical reaction wherein the average particle diameter of the gasgenerating agent is below 100 μm in diameter, in the preparation of aparticulated ammonium nitrate product to form a particulated ammoniumnitrate product with reduced density.

The invention also relates to a particulated ammonium nitrate productwith reduced density prepared according to the method substantially asdescribed herein above. The particulated ammonium nitrate may compriseporous product such as PPAN (porous prilled ammonium nitrate) or porousgranulated ammonium nitrate.

The invention also relates to a blasting composition includingparticulated ammonium nitrate substantially as described hereinabove.The blasting composition may comprise ANFO or heavy ANFO.

The invention will now be further described by means of the followingnon-limiting examples.

EXAMPLE 1

A three hour test run in a conventional prilling plant where PPAN isproduced from an ammonium nitrate (AN) solution was conducted byintroducing a gas generating agent into the AN to be prilled.

The prilling plant is a conventional one and is accordingly notdescribed in detail in this specification. Some more detail of theapparatus used is described in U.S. Pat. No. 5,540,793. Amongst othersthe apparatus comprises a prilling nozzle for forming prills.

Gas generating agent in the form of precipitated calcium carbonate(CaCO₃) with a particle diameter of about 0.33 μm was made up as anaqueous suspension comprising 30% mass CaCO₃/mass aqueous suspension. Ithas been found that if large particles of CaCO₃ are used in an ANsolution, the pH of the AN solution has to be lowered to provide anadequate rate of gas generation. If the pH of AN is lowered too much,rendering it less stable, the decomposition of AN increases.

The AN to be prilled comprised an aqueous AN solution comprising 97%mass AN/mass AN solution. The solution was provided at a temperature of158° C. The pH of an AN solution was lowered from the normal pH of 5.8to a pH of 3.8 during the test run to increase the rate of gassing. ThepH of the AN solution was adjusted by reducing the addition of ammoniaafter the concentration step.

The calcium carbonate suspension was added in order that the AN solutioncontained about 0.03% mass calcium carbonate/mass total AN solution. Thecalcium carbonate was added in order that it was retained in the ANsolution for a period of about 2 seconds before the formation of theammonium nitrate into droplets which formed into the prills.

Table 1 shows the results of the tests on the PPAN produced compared toPPAN produced under similar conditions but without the addition ofCaCO₃.

In Table 1 the percentage of CaCO3 is expressed as percentage massCaCO₃/mass of PPAN solution. The volume of closed pores or voids wascalculated from the apparent and true particle densities. The porevolume was determined with a mercury porosity meter, which determinespore value as a function of the mercury pressure and consequently of thepore size. The apparent particle density is determined by measuring thevolume change when 50 grams of prills are carefully added to 50 mldiesel in a 100 ml measuring cylinder. A stopper is placed on themeasuring cylinder and the cylinder is shaken and tapped to allow thediesel to penetrate most of the open pores. The effectively closed porescan then be calculated from the apparent particle density and the trueparticle density. The formula is: closed pores=(1/apparent particledensity)−1/1.724 (the true particle density of ammonium nitrate at roomtemperature is 1 1.724 g/cc). The oil absorption capacity is expressedas percentage mass oil/mass prills.

The fuelled prills were formed by adding diesel in an amount of 6% massdiesel/mass ANFO.

TABLE 1 Test Results PPAN with Normal PPAN CaCO₃ without CaCO₃ Calcium(as CaCO₃) [[0,03]] 0.03  0 Loose bulk density (kg/l) [[0,67]] 0.67[[0,74]] 0.74 Apparent particle density (g/ml) [[1,41]] 1.41 [[1,50]]1.50 Closed pores (ml/g) [[0,13]] 0.13 [[0,08]] 0.08 Pore volume (20 to70 μm; ml/g) [[0.057]] 0.057 [[0,013]] 0.013 Oil absorption capacity (%)10 10 Sensitivity of fuelled prills 49  1 (6% m/m diesel in ANFO. Leadcompression in mm)From Table 1 it can be seen that gassing with CO₂ formed in situ fromCaCO₃ reacting with nitric acid in the AN solution resulted in:

-   a) A significant increase in the volume of closed pores and    consequently a decrease in the loose bulk density; and-   b) A more sensitive fuelled prill (ANFO), as indicated by the lead    compression test. The increased sensitivity was probably caused by    an increase in the volume of small (20 to 70 μm) closed pores. These    pores must be closed or the opening so small that diesel is not    absorbed.

EXAMPLE 2

A pan granulator was used to form AN granules with a reduced density byintroducing a gas generating agent into the AN.

The AN to be particularized comprised 99.5% AN and 0.5% water. The ANwas provided in the form of a melt at about 170° C. and the pH of the ANwas 3.8.

The gas generated agent was again in the form of precipitated CaCO₃ witha particle diameter of about 0.33 μm.

A pan granulator and seed material (in the form of particulated ammoniumnitrate of a suitable size) were used to form the AN melt into granules.In test 1, the seed material was introduced into the pan granulator andwas sprayed with the AN melt which formed into granules. In test 2 theseed material was mixed with the CaCO₃. This mixture was introduced intothe pan granulator and sprayed with the AN melt. The granulated AN whichformed contained about 0.1% (mass/mass) CaCO₃. In test 3 the CaCO₃ wasmade up as an aqueous suspension containing 20% mass CaCO₃/mass aqueoussuspension. The CaCO₃ was injected into the AN melt which mixture wasthen sprayed onto the seed material in the pan granulator to formgranules. The AN melt sprayed onto the seed material contained about0.1% mass CaCO₃/mass total AN melt. The CaCO₃ was added in order that itwas retained in the AN melt for a period of about 2 seconds before beingsprayed onto the seed material.

Table 2 shows the results of the granulated AN formed by the differenttests. Percentages are expressed as mass per mass.

TABLE 2 TEST RESULTS Test 1 Test 2 Test 3 Moisture % [[0,17]] 0.17[[0,16]] 0.16 [[0,20]] 0.20 Oil absorption capacity % 7 8 10 Loose bulkdensity (kg/l) [[0,66]] 0.66 [[0,62]] 0.62 [[0,62]] 0.62

From table 2 it is clear that the use of the gas generating agentprovided a product with increased oil absorption capacity and a reducedbulk density.

By using a gas generating agent such as precipitated CaCO₃ moreconcentrated An solution can be used to prepare explosive grade ammoniumnitrate than used in the normal prilling process. This allows for theproduction of porous AN or AN with reduced density by use of a pangranulator, blunger, drum granulator or other granulation methods.

It will be appreciated that many variations in detail are possiblewithout thereby departing from the scope and spirit of the invention.

1. A method of reducing density in an ammonium nitrate productcomprising: providing a gas generating agent in the form of awater-insoluble solid compound capable of generating gas by chemicalreaction with a suitable acid and wherein the average particle diameterof the gas generating agent is below 40 μm; providing an ammoniumnitrate product which is acidic with a pH between 2 and 5; introducingthe water-insoluble solid gas generating agent into the ammonium nitrateproduct under conditions causing gas generation by the gas generatingagent in the ammonium nitrate product; and particulating the resultingammonium nitrate product to form a particulated ammonium nitrate productwith reduced density.
 2. The method of claim 1 wherein the conditionscausing gas generation in the ammonium nitrate product are controlled toregulate the rate of gas generation and bubble size of the generatedgas.
 3. The method of claim 2 wherein the conditions are controlled bycontrolling at least one of the conditions selected from the groupconsisting of the type of gas generating agent added; amount of gasgenerating agent added; particle diameter of the gas generating agentadded; pH of the reaction mixture of the gas generating agent andammonium nitrate product; pH of the ammonium nitrate product; type ofacid provided in the ammonium nitrate product; concentration of acidprovided in the ammonium nitrate product; type of base provided in theammonium nitrate product; concentration of base provided in the ammoniumnitrate product; the temperature at which the gas generation reactiontakes place; and the concentration of the ammonium nitrate product. 4.The method of claim 1 wherein the gas generating agent comprises awater-insoluble carbonate compound.
 5. The method of claim 4 wherein thecarbonate compound comprises an alkaline earth metal carbonate.
 6. Themethod of claim 5 wherein the carbonate compound comprises calciumcarbonate.
 7. The method of claim 4 wherein the carbonate compound is acompound capable of generating carbon dioxide by chemical reaction witha suitable acid.
 8. The method of claim 1 wherein the average particlediameter of the water-insoluble gas generating agent is below 2 μm. 9.The method of claim 8 wherein the average particle diameter of thewater-insoluble gas generating agent is below 0.5 μm.
 10. The method ofclaim 1 wherein the water-insoluble gas generating agent is calciumcarbonate with an average particle diameter of about 0.33 μm.
 11. Themethod of claim 10 wherein the calcium carbonate is precipitated calciumcarbonate.
 12. The method of claim 11 wherein the calcium carbonate isprovided in the form of an aqueous suspension.
 13. The method of claim11 wherein the calcium carbonate is added to the ammonium nitrateproduct at a concentration of 0.001% to 2% mass calcium carbonate/massammonium nitrate product.
 14. The method of claim 1 wherein the ammoniumnitrate product comprises a melt of ammonium nitrate.
 15. The method ofclaim 1 wherein the ammonium nitrate product comprises an aqueoussolution of ammonium nitrate.
 16. The method of claim 1 wherein theammonium nitrate product is acidic with a pH between 2 and
 5. 17. Themethod of claim 1 wherein hollow microspheres are also introduced intothe resulting ammonium nitrate product.
 18. The method of claim 1wherein the resulting ammonium nitrate product is particulated by meansof prilling.
 19. The method of claim 1 wherein the resulting ammoniumnitrate product is particulated by means of granulation.
 20. Theparticulated ammonium nitrate product produced by the method of claim 1.21. A blasting composition including a particulated ammonium nitrateproduct of claim 20.